Southwest Florida manatee death toll has likely peaked

(News Press) -- Nobody's breaking out the Champagne, but the worst of Southwest Florida's record red tide-related manatee die-off might be over.

Through April 12, 290 manatees have died from Sarasota County to Collier County, 244 of them in Lee County.

Of the total Southwest Florida manatee deaths, 264 were either caused by red tide or are red tide suspects (a manatee death is a red tide suspect when its carcass is found in a red tide area, but researchers can't determine whether red tide killed the animal.).

A single-day high 16 manatees were documented March 4, 13 of them in Lee County.

From Jan. 15 to April 12, the longest period during which no dead manatees were documented in the area was three days.

About five dead manatees have been documented in Southwest Florida since then - an exact number was not available last week, said Kevin Baxter, spokesman for the Florida Fish and Wildlife Research Institute.

"It's definitely slowed down in the last couple of weeks," he said. "In our past experience with red tide, even when the bloom has dissipated, we see dead manatees for a couple of months afterward."

Red tide, a natural phenomenon caused by the single-cell alga Karenia brevis, which produces a powerful neurotoxin, showed up on Southwest Florida waters during September 2012.

Area waters have been nearly free of Karenia for more than a month, but manatees have continued to die (71 between March 20 and April 12) because they're eating seagrass that absorbed red tide toxin.

Dead manatees during the die-off have been found in 35 water bodies from Little Sarasota Bayto Rookery Bay in Collier County.

More manatee carcasses (68) have been recorded in the Caloosahatchee River than any other water body.

The Orange River ranked second in manatee carcasses with 54, followed by Matlacha Pass with 47 - the Caloosahatchee and Orange rivers and Matlacha Pass account for 68 percent of the total carcasses.

Researchers don't know why so many manatees have died in those three water bodies, said Kipp Frohlich, leader of the state's Species Management Section.

"Obviously, this event is still under way, and we're trying to keep up with it," he said. "I'm not sure there are easy answers to some of these questions.

"It could be that more manatees die where there are more manatees. Or more die where the red tide concentrations are higher. Sometimes it's not the concentration, but how it's concentrated."

So far, 132 dead manatees have been adults.

"In most mammal populations, the death of adults is considered to have a larger impact than the deaths of juveniles," Frohlich said. "The reason is that adults are reproducing, adding to the population. In the case of manatees, since they're such slow reproducers and so long-lived and an endangered species, we don't want to lose any."

Almost half the dead manatees (130) were males; 122 were females and 12 were too decomposed for sex to be determined.

"There's no pair-bond in manatees: It doesn't take two manatees to raise a calf, just the female," Frohlich said. "From that breeding strategy, females are usually considered more valuable than males."

Florida's manatees have suffered several unusual mortality events dating back to the 1990s: Red tide killed 151 manatees in 1996, 100 in 2003 and 93 in 2005; cold stress killed 72 manatees during the winter of 2008 and 2009 and 480 manatees from Jan. 11 through April 9, 2010.

According to the state's 2007 Florida Manatee Management Plan, there are fewer than 2,500 mature adult manatees in Florida, and, while populations in other parts of the state are growing, the Southwest population is decreasing by an estimated 1.1 percent a year.

How this year's red tide die-off will affect the manatee population remains to be seen, Frohlich said.

"The data collectors and statisticians will tell us what it means," he said. "But it stands to reason when you have big death events, like cold stress and now this, that it will have an impact. It might take a couple of years to see whether it affects the population in terms of negative growth or slowing positive growth."